Error on Page Return to Login Page

[#1]

Hi  Not been with Plusnet long, maybe a few weeks.  But when I receive SPAM or just emails I wish to delete, I toggle them and hit delete or SPAM and it immediately tells me Error On Page! Return to Login.
Why is this?  And how can I stop it?

[#2]

See: Plusnet crawl with Webmail

[#3]

Is the error along the lines of this?

ERROR:
ERROR: Could not append message to .
Server responded: Error in IMAP command received by server.
ERROR:
ERROR: Could not append message to INBOX.Sent.
Server responded: Error in IMAP command received by server.

If so, then it's a known problem (ref: 57901).
FWIW, there are two places you can click the spam button. On the message list view by selecting the check box next to a particular message, or by clicking the email itself and then clicking the spam button in the header of the message.
The second option generates an error whereas the first one doesn't.